In this kind of vacuum belt filter, the belt or cloth is stationary whilst the slurry is being subjected to suction, through the belt or cloth, by the vacuum box arrangement, and the belt or cloth is stepped forward whilst the suction is switched off. The stepping forward of the belt or cloth may be effected by various means such as an air motor, an electric motor, or a rotating drive arrangement. More recently I have developed an arrangement wherein a pneumatic ram or like mechanism serves to displace a front guide roller for the forward end of the upper run of the belt so as to extend such upper run by drawing a further portion of the belt or cloth into its rearward end, this being permitted by a section of the belt or cloth formed into a reserve bight by a weighted roller. In this arrangement, reverse movement of the belt or cloth is prevented by a unidirectional mechanism acting on the belt or cloth in a return run thereof before it reaches the reserve bight.
An advantage of this proposal involving a ram and a unidirectional mechanism (such as a free-wheel roller which is permitted to rotate in one direction only) lies in the fact that no additional drive for the belt or cloth has to be provided over and above the pneumatic ram or like arrangement which extends the forward end of the upper run of the belt or cloth.
A potential disadvantage thereof, however, lies in the fact that the weighted roller does not only impart tension to the belt or cloth and maintain the reserve bight. It has to be heavy enough to be effective, upon reverse movement of the front guide roller, to cause the corresponding forward end portion of the upper run of the belt or cloth to be drawn into the return run, and a corresponding portion of the latter to be drawn into the reserve bight. The weighted roller may, accordingly, have to be very heavy, with the result that it imposes an unnecessarily high tension on the cloth. This can be particularly the case where a scraper or like member is used, upstream of the upper run of the belt or cloth, for removing the filter cake from the cloth, as considerable friction can arise with such a scraper and this friction has to be overcome in moving the belt or cloth. In some instances, the forces involved may be so great that the use of a weighted roller is impracticable.
An object of the present invention is to provide an arrangement whereby these difficulties are overcome, the means for drawing the belt forwards stepwise in its upper run correspondingly positively displacing the belt along at least part of its return run, so that it will do any necessary work in overcoming the friction of a scraper or like mechanism (if provided) for removing filter cake and it largely relieves the weighted roller of the task of displacing the belt, thereby leaving such weighted roller to function principally to impart tension to the belt or cloth and to compensate for any irregularities in the movement of the belt or cloth.